This invention relates to a corrosion inhibitor and the use thereof in wellbore applications.
Aerated liquids and foams are frequently employed as wellbore fluids during drilling, completion, workover and production operations. Aerated as used herein refers to the existence of distinct gas and liquid phases. Aerated liquids are distinguishable from foams by the absence or the presence in de minimis amounts of surface active agents which promote gas-liquid interphase dispersibility.
Aerated liquids and foams are particularly useful when reduced hydrostatic pressures in wellbores are desired. The reduced hydrostatic pressure or head is made possible by the lower bulk densities of aerated liquids and foams when compared to their liquid counterparts. Examples wherein reduced hydrostatic heads are favored include (1) situations where the wellbore fluids are exposed to under-pressured geological strata and minimum invasion of the strata by the wellbore fluid is desired and (2) during the perforation of a geological strata at underbalanced conditions (i.e., wellbore pressure less than fluid pressure in exposed strata) wherein the reduced head helps to minimize plugging of the perforation caused by movement of fines into the perforation from the wellbore.
Aerated liquids are nominally comprised of a gas and a liquid and may additionally contain additives such as corrosion inhibitors and suspended solids such as mud and sand. Aerated fluids useful in drilling, completion, workover and production operations are typically prepared by the downhole combination of a gas and a liquid. In drilling, one means of combining these components downhole is by the use of dual drill strings and dual swivels. Another means frequently used in downhole applications is to employ parasitic tubing for transporting the gas downhole to an injection point where it is combined with liquid and returned to the surface. Injection of gas at any downhole location serves to reduce the bottomhole hydrostatic head from that of a 100% liquid head. Downhole gas injection is also practiced when gas lift is used to facilitate fluids production in a well.
Foams are highly dispersed gas in liquid two-phase systems. A foam is physically composed of gas bubbles surrounded by a surfactant-bearing aqueous film and is nominally comprised of a gas, at least one surfactant which functions as a foaming agent, and water. The quality of the foam or foam quality is defined to be the volume percent of gas in the two-phase mixture. The water used in foams may be chemically pure, fresh, or contain varying degrees of salinity and/or hardness. The foam may additionally be comprised of other additives which alter the properties of the foam. Such additives include selected polymers and/or bentonite which increase film strength thereby forming stiff or gel foams and corrosion inhibitors which function to decrease foam corrositivity in metal-bearing systems.
The rheological properties of foam are complex and provide a separate and distinct basis for distinguishing foam from aerated and non-aerated liquids. The rheological properties are dependent upon many parameters such as bubble size and distribution, fluid viscosity, foam quality, and the concentration and type of foaming agent used. The use of foam in place of conventional fluids generally reduces the degree of fluid invasion from the wellbore into the surrounding strata. Furthermore, the foam which does invade the strata generally contains a high volume percentage of gas which upon pressurization possesses significant energy. Upon depressurization, the stored energy which is released causes a significant portion of this fluid to be returned to the wellbore. Compared to conventional fluids, foam also possesses at low linear flow velocities excellent carrying capacities, particularly for water and solids. These properties are particularly useful when conducting drilling, completion and workover operations on vertical and horizontal wells and even more so when low pressure, semi-depleted or water sensitive formations are encountered. The unique properties of foam also enable the use of coiled tubing units during workover operations. The use of such units results in significant savings of time and money because downhole operations can be performed without removal of the wellbore tubing.
Historically, a major problem associated with the use of aerated liquids and foams has been metal corrosion attributed to the presence of oxidizing agents such as oxygen, hydrogen sulfide and carbon dioxide in the gas phase. Oxygen generally originates from air. Carbon dioxide and hydrogen sulfide are generally produced from the geological strata of interest. Corrosion problems generally become more sever as wellbore temperatures increase. Because of cost and availability, air is the preferred gaseous species when downhole corrosion is not a problem. However, because of the lack of suitable corrosion inhibitors when downhole conditions become severe (i.e., high temperatures and the presence of significant quantities of oxidizing agents), inert gases which are expensive, most notably nitrogen, are frequently used.